1. Field of the Invention
The present invention relates to a magneto-resistive effect device for reading the magnetic field strength of a magnetic recording medium or the like as signals, a thin-film magnetic head comprising that magneto-resistive effect device, and a head gimbal assembly and a magnetic disk system comprising that thin-film magnetic head.
2. Explanation of the Prior Art
With recent improvements in the plane recording density of magnetic disk systems, there have been growing demands for improvements in the performance of thin-film magnetic heads. For the thin-film magnetic head, a composite type thin-film magnetic head has been widely used, which has a structure wherein a reproducing head having a read-only magneto-resistive effect device (hereinafter often referred to as the MR (magneto-resistive) device for short) and a recording head having a write-only induction type magnetic device are stacked on a substrate.
For the MR device, there is the mention of an AMR device harnessing an anisotropic magneto-resistive effect, a GMR device tapping a giant magneto-resistive effect, a TMR device making use of a tunnel-type magneto-resistive effect, and so on.
The reproducing head is required to have high sensitivity and high outputs in particular. GMR heads using a spin valve type GMR device have already been mass-produced as a reproduction head possessing such performances, and to meet further improvements in plane recording densities, reproducing heads using TMR devices are now being mass-produced, too.
In general, the spin valve type GMR device comprises a nonmagnetic layer, a free layer formed on one surface of that nonmagnetic layer, a fixed magnetization layer formed on another surface of the nonmagnetic layer, and a pinning layer (generally an antiferromagnetic layer) on the side of the fixed magnetization layer facing away from the non-magnetic layer. The free layer has its magnetization direction changing depending on an external signal magnetic field, and the fixed magnetization layer has its magnetization direction fixed by a magnetic field from the pinning layer (antiferromagnetic layer).
And now, common GMR heads used so far in the art have a CIP (current in plane) structure wherein a current for detecting magnetic signals (the so-called sense current) is passed parallel with the plane of each of the layers forming the GMR device (CIP-GMR device). On the other hand, GMR devices having the so-called CPP (current perpendicular to plane) structure wherein the sense current is passed perpendicularly to the plane of each of the layers forming the GMR device (CPP-GMR device), too, are now under development as next-generation ones.
The aforesaid TMR devices, too, would come under the CPP structure category according to a classification system from the current-passing direction alone. However, the multilayer construction and detection principle of the TMR device are different from those of the CPP-GMR device. That is, the TMR device generally comprises a free layer, a fixed magnetization layer, a tunnel barrier layer located between them, and an antiferromagnetic layer located on the plane of the fixed magnetized layer that faces away from its plane in contact with the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic insulating layer through which electrons can pass in a state with spins reserved by the tunnel effect. The rest of the multilayer structure, i.e., the free layer, fixed magnetization layer and antiferromagnetic layer could be basically identical with those used with the spin valve type GMR device.
It is here noted that when the TMR device is used for a reproducing head, it is required to have low resistance for the following reasons. For a magnetic disk system, there is a demand for improved recording density and improved data transfer rate, with which the reproducing head is required to have good high-frequency response. However, as the resistance value of the TMR device grows large, it will cause an increase in stray capacitances occurring at the TMR device and a circuit connected to it, rendering the high-frequency response of the reproducing head worse. This is the reason the TMR device must inevitably have low resistance.
Generally speaking, reducing the thickness of the tunnel barrier layer would work for making the resistance of the TMR device low. However, too thin a tunnel barrier layer would cause a lot more pinholes to occur in the tunnel barrier layer, rendering the service life of the TMR device short. Further, there would be a magnetic couple produced between the free layer and the fixed magnetization layer, ending up with problems: a lot more noise, a drop of the MR ratio, and degradation of TMR device's performance. The noise occurring at the reproducing head is here called head noise. The head noise occurring at the reproducing head using the TMR device includes shot noise, a noise component that is unlikely to occur at a reproducing head using the GMR device. Thus, a problem with the reproducing head using the TMR device is that the head noise is noticeable.
With the CPP-GMR device, on the other hand, there is a problem that no large enough MR ratio is obtained. A possible reason for it could be that spin-polarized electrons are scattered at the interface between the nonmagnetic electroconductive layer and the magnetic layer, and in the nonmagnetic electroconductive layer.
Also, the CPP-GMR device, because of having a small resistance value, is low in terms of the amount of resistance change. For this reason, in order to obtain large reproduction output with the CPP-GMR device, high voltage must be applied to that device. However, the application of high voltage to the device offers such problems as described below. With the CPP-GMR device, currents are passed in the direction perpendicular to the plane of each layer, whereupon spin-polarized electrons are poured from the free layer into the fixed magnetization layer or from the fixed magnetization layer into the free layer. Such spin-polarized electrons cause torque (hereinafter called the spin torque) that rotates those magnetizations to be generated at the free layer or the fixed magnetization layer. The magnitude of this spin torque is proportional to the current density. As the voltage applied to the CPP-GMR device grows high, it causes the current density to grow large with the result that there is large spin torque. As the spin torque increases, there are problems such as changes in the direction of magnetization of the fixed magnetization layer, and the inability of the free layer to freely change the direction of magnetization with respect to an external magnetic field.
To solve such problems, Applicant has already filed Japanese Patent Application No. 2006-275972 to come up with an invention relating to a CPP-GMR device, with which large MR ratios are achieved while noise is held back and the influence of the spin torque is reduced.
That is to say, in a preferable embodiment of that invention, a spacer layer interposed between the free layer and the fixed magnetized layer has typically a Cu/ZnO/Cu multilayer structure, and the area resistivity of a magneto-resistive effect device and the electro-conductivity of the spacer layer are determined in such a way as to fall within the range of 0.1 to 0.3Ω·μm2 and the range of 133 to 432 S/cm, respectively.
By allowing the spacer layer to have typically a three-layer structure of Cu/ZnO/Cu according to this proposal, large MR ratios are achievable while holding back noise and reducing the influence of the spin torque.
The present invention is an invention for making improvements in or relating to Japanese Patent Application No. 2006-275972, and embodied as follows.
That is, when, in Japanese Patent Application No. 2006-275972, it is intended to limit the area resistivity and electroconductivity of the device within the predetermined ranges, the semiconductor layer represented by ZnO and used as the intermediate layer of the triple-layer structure of the spacer layer must be as thin as about 1.2 to 2.0 nm. A semiconductor layer having a thickness of greater than 2.0 nm does not only cause the area resistivity of the device to grow too large, but also there is a tendency for shot noise to occur as is the case with the tunnel barrier (TMR) using an ordinary insulating layer or semiconductor layer. As a result, even at an improved MR ratio, a noise component tends to grow large, resulting in a worsening of the S/N.
Of course, there is no problem with the use of a thickness of as small as about 1.2 to 2.0 nm, if devices are produced under severe production conditions and quality control. However, as the semiconductor layer represented by ZnO is thin, there are a lot more pinholes occurring due to thickness variations on film formation, which may possibly give rise to a worsening of characteristics reliability due to such electromigration as found in the so-called current narrowing type CPP-GMR. Further, on polishing, there may possibly be a short circuit occurring in Cu layer on each side of ZnO, which may further bring about phenomena: the occurrence of noises and the deterioration of the MR change rate. Still further, there may be disturbances in the crystal lattices of ZnO with varying film-formation conditions such as partial oxygen pressure, and the larger the thickness, the more often this would occur; with decreasing thickness, the volume fraction of a portion affected by lattice disturbances increases.
Thus, with design specifications where the semiconductor layer used as the intermediate layer of the triple-layer structure of the spacer layer and represented by ZnO grows thin, a variety of troubles may possibly be caused.
Therefore, with regard to the semiconductor layer used as the intermediate layer of the triple-layer structure of the spacer layer, there is a mounting demand for specifications where that layer can be reduced as much as possible while the area resistivity of the device is kept low as desired and an increase in the noise component (the shot noise occurring upon tunnel conduction in particular) is prevented.
Note here that the prior art that seems to be most relevant to the invention of this application is JP-A-2003-8102. This prior art sets forth a CPP-GMR device comprising a fixed magnetization layer having a fixed magnetization direction, a free magnetization layer with its magnetization direction changing depending on an external magnetic field, a nonmagnetic metal intermediate layer interposed between the fixed magnetization layer and the free magnetization layer, and a resistance control layer interposed between the fixed magnetization layer and the free magnetization layer and formed of a material having conduction carriers of up to 1022/cm3. The prior art shows a semiconductor as one of resistance control layer materials; however, it does not suggest at al about the requirements for the invention of this application.